1. Field of the Invention
This invention relates to the fabrication of microstructures, and more particularly to a method of making an active micro-fluidic device from a micro-machined substrate.
2. Description of Related Art
Micro-fluidic devices are used in many applications. They come in two varieties: active and passive. Typical examples of active devices would be micro-detection/analysis/reactor systems; micro-chemical detection/analysis/reactor systems; micro-opto-fluidics systems; micro-fluid delivery systems; micro-fluid interconnect systems; micro-fluid transport systems; micro-fluid mixing systems; micro-valves/pumps systems; micro flow/pressure systems; micro-fluid control systems; micro-heating/cooling systems; micro-fluidic packaging; micro-inkjet printing; biochips; and laboratory-on-a-chip, LOAC, devices. Typical examples of passive (i.e. off-chip electronics) micro-channels, would be micro-chemical detection/analysis systems; micro-detection/analysis systems; micro-chemical detection/analysis systems; micro-opto-fluidics systems; micro-fluid delivery systems; micro-fluid interconnect systems; micro-fluid transport systems; micro-fluid mixing systems; micro-valves/pumps systems; micro flow/pressure systems; micro-fluid control systems; micro-heating/cooling systems. micro-fluidic packaging; micro-inkjet printing; biochips; and LOAC devices.
The prior art shows that passive micro-fluidics devices with micro-channels are largely fabricated from the combination of various polymer substrates, such as: acrylonitrile-butadiene-styrene copolymer, polycarbonate, polydimethylsiloxane (PDMS), polyethylene, polymethylmethacrylate (PMMA), polymethylpentene, polypropylene, polystyrene, polysulfone, polytetrafluoroethylene (PTFE), polyurethane, polyvinylchloride (PVC), polyvinylidine fluoride (PVF), or other polymer. In this case, lithography or mechanical stamping is used to define a network of micro-fluidics channels in one of these substrate, prior to the assembly and the thermally assisted bonding of this first substrate to another such substrate. The result is a simple passive micro-fluidics device which can be patterned with conductive layers as to connect an external processor used to provoke fluid movement by electrophoresis or electro-osmosis, analysis and data generation. FIGS. 1a to 1c of U.S. Pat. No. 6,167,910 show an example of a passive micro-fluidics device obtained from the fusion of such polymeric substrates.
The prior art also indicates that passive micro-fluidics devices with micro-channels can be fabricated by combining various micro-machined silica or quartz substrates. Again, assembly and fusion bonding is required. The result is a simple passive micro-fluidics device which can be patterned with conductive layers as to connect an external processor used to provoke fluid movement by electrophoresis or electro-osmosis, analysis and data generation. FIGS. 1 of U.S. Pat. No. 6,131,410 shows an example of such passive micro-fluidics devices with micro-channels obtained from the fusion of such silica substrates.
The prior art indicates that passive micro-fluidics devices with micro-channels can be fabricated from a passive micro-machined silicon structural substrate. Again, assembly and fusion bonding of at least two sub-assemblies is required. The result is a simple passive micro-fluidics device to connect to an external processor used to provoke fluid movement, analysis and data generation. FIGS. 1 to 3 of U.S. Pat. No. 5,705,018 show an example of such passive micro-fluidics devices with micro-channels obtained from a passive micro-machined silicon substrate.
The prior art also discloses that active micro-fluidics devices (with no micro-channels) can be fabricated from an active micro-machined silicon substrate. In this case, the control electronics integrated in the silicon substrate is used as an active on-chip fluid processor and communication device. The result is a sophisticated device which can perform, in pre-defined reservoirs, without micro-channels, various fluidics, analysis and (remote) data communication functions without the need of an external fluid processor in charge of fluid movement, analysis and data generation. FIG. 3B of U.S. Pat. No. 6,117,643 shows an example of such active micro-fluidics devices (with no micro-channels) obtained from an active micro-machined silicon substrate.
This invention relates to an improved micro-machining technique which uses a maximum processing temperature of less than 500xc2x0 C. to fabricate micro-fluidics elements and micro-channels over an active semiconductor device thus resulting in integrated active micro-fluidics devices with micro-channels. The manufacturing of micro-fluidic devices with micro-channels requires the fabrication of micro-fluidics elements and micro-channels for the processing of fluids.
Accordingly the present invention provides a method of fabricating a microstructure for micro-fluidics applications, comprising the steps of forming a layer of etchable material on a substrate; forming a mechanically stable support layer over said etchable material; performing an anisotropic etch through a mask to form a pattern of holes extending through said support layer into said etchable material, said holes being separated from each other by a predetermined distance; performing an isotropic etch through each said hole to form a corresponding cavity in said etchable material under each said hole and extending under said support layer; and forming a further layer of depositable material over said support layer until portions of said depositable layer overhanging each said hole meet and thereby close the cavity formed under each said hole.
The holes should generally be set a distance apart so that after the isotropic etch the cavities overlap to form the micro-channels. In one embodiment, they can be set further apart so as to form pillars between the cavities. This embodiment is useful for the fabrication of micro-filters.
The invention permits the fabrication of active micro-fluidics devices with micro-channels from an active micro-machined silicon substrate directly over a Complementary Metal Oxide Semiconductor device, CMOS device, or a high-voltage CMOS (or BCD) device.
CMOS devices are capable of very small detection levels, an important prerequisite in order to perform electronic capacitance detection (identification) of entities in suspension in the fluids with low signal levels. CMOS devices can perform the required data processing and (remote) communication functions. High-voltage CMOS (or Bipolar-CMOS-DMOS, BCD) devices with adequate operation voltages and operation currents are capable of performing the required micro-fluidics in the micro-channels and allowing the integration of a complete Laboratory-On-A-Chip concept.
This invention employs an improved micro-machining technique used to integrate to CMOS and high-voltage CMOS (or BCD) devices the micro-machining steps which allow the fabrication of the micro-fluidics elements and micro-channels at a maximum processing temperature not exceeding 500xc2x0 C. without the use of a second substrate and without the use of thermal bonding. The maximum processing temperature of 500xc2x0 C. prevents the degradation of the underlying CMOS and high-voltage CMOS (or BCD) devices; and prevents any mechanical problems such as plastic deformation, peeling, cracking, delamination and other such high temperature related problems with the thin layers used in the micro-machining of the micro-fluidics device.
The novel materials combination described is not typical of Micro-Electro-Mechanical-Systems (MEMS) which typically use Low Pressure Chemical Vapour Deposited polysilicon, LPCVD polysilicon, and Plasma Enhanced Chemical Vapour Deposited silica, PECVD SiO2, combinations. The use of LPCVD polysilicon is proscribed because of its required deposition temperature of more then 550xc2x0 C.
An innovative sacrificial material is Collimated Reactive Physical Vapour Deposition of Titanium Nitride, CRPVD TiN. This sacrificial CRPVD TiN material has excellent mechanical properties, excellent selectivity to Isotropic Wet Etching solutions used to define the micro-channels in thick layers of Plasma Enhanced Chemical Vapour Deposited, PECVD, SiO2, and a deposition temperature of about 400xc2x0 C.